The present invention is related to capacitive pressure transducers. More particularly, the present invention relates to controlling deposition of contaminants in capacitive pressure transducers.
FIG. 1A shows a sectional side view of a prior art capacitive pressure transducer 100. For convenience of illustration, FIG. 1A, as well as other figures in the present disclosure, are not drawn to scale. As shown, transducer 100 includes a housing 102, a capacitive pressure sensor 106 disposed within housing 102, an inlet tube 104, and a filtering mechanism 108. For convenience of illustration, many details of transducer 100 are omitted from FIG. 1A. However, such sensors are well known and are described, for example, in U.S. Pat. Nos. 5,911,162 and 6,105,436 and U.S. patent application Ser. Nos. 09/394,804 and 09/637,980.
Briefly, transducer 100 is normally coupled to a gas line 110, or some other external source of gas or fluid 11 by a coupling 112. In operation, sensor 106 generates an output signal representative of the pressure of gas 111 (i.e., the pressure within external source 110).
Pressure transducers such as transducer 100 are often used in integrated circuit fabrication foundries, for example, to measure the pressure of a fluid in a gas line that is being delivered to a deposition chamber, or to measure the pressure within the deposition chamber itself. Some of the processes used in integrated circuit fabrication, such as the etching of aluminum, tend to generate a large volume of particles or contaminants. It is generally desirable to prevent such contaminants from entering the sensor 106. When contaminants do enter, or become built up in, sensor 106, the accuracy of the pressure measurement provided by transducer 100 is adversely affected. Accordingly, prior art pressure transducers have used a variety of mechanisms to prevent contaminants from reaching the sensor 106. Such prior art filtering mechanisms are generally disposed between the inlet tube 104 and the sensor 106, and are indicated generally in FIG. 1A at 108.
FIG. 1B shows a more detailed view of a particular prior art pressure transducer 100 showing both the sensor 106 (which as discussed below includes elements 127a, 127b, and 160) and the filtration mechanisms 108 (which as discussed below includes elements 140, 150). Transducer 100 includes a lower housing 102a and an upper housing 102b, which are separated by a relatively thin, flexible conductive diaphragm 160. Lower and upper housings 102a, 102b, and diaphragm 160 are normally welded together. Upper housing 102b and diaphragm 160 define a sealed interior chamber 120. Lower housing 102a and diaphragm 160 define an interior chamber 130 that opens into inlet tube 104. Diaphragm 160 is mounted so that it flexes, or deflects, in response to pressure differentials in chambers 120, 130.
Transducer 100 includes a ceramic electrode 122 disposed within chamber 120. Electrode 122 is supported within chamber 120 by a support 124. An inner conductor 127a and an outer conductor 127b are disposed on the bottom of electrode 122. FIG. 1C shows a bottom view of electrode 122 showing the geometries of the inner and outer conductors 127a, 127b. As shown, inner conductor 127a is circular. Outer conductor 127b is annular and surrounds inner conductor 127a. The area of inner conductor 127a is normally selected to be equal to the area of outer conductor 127b. Conductors 127a, 127b are generally parallel to and spaced apart from diaphragm 160. Diaphragm 160 and the conductors 127a, 127b form two variable capacitors 128a, 128b. More particularly, diaphragm 160 and inner conductor 127a form a variable inner capacitor 128a, which is characterized by an inner capacitance Cinner. Similarly, diaphragm 160 and outer conductor 127b form a variable outer capacitor 128b, which is characterized by an outer capacitance Couter.
The capacitance of each of the variable capacitors is determined, in part, by the distance d between the diaphragm and the relevant conductor. More specifically, as is well known, C=Aere0/d, where C is the capacitance between two parallel conductive plates, A is the common area between the plates, e0 is the permittivity of a vacuum, er is the relative permittivity of the material separating the plates (er=1 for vacuum), and d is the axial distance between the plates (i.e., the distance between the plates measured along an axis normal to the plates).
As diaphragm 160 flexes in response to changes in the differential pressure between chambers 120, 130, the capacitances of the variable capacitors 128a, 128b change and thereby provide an indication of the differential pressure.
A reference pressure, which may be near vacuum, is normally provided in chamber 120, inlet tube 104 is connected via coupling 112 to a gas line 110 containing gas 111, and transducer 100 provides an electrical output signal indicative of the pressure of gas 111. In other configurations, a second inlet tube leading into chamber 120 may be provided and connected to a second external source. In such configurations, transducer 100 provides a signal indicative of the differential pressure between the two external sources. Transducers will be discussed herein in the context of measuring the pressure of gas or fluid 111, but it will be appreciated that they can also be used as differential pressure transducers.
A capacitive pressure transducer can be built using only a single conductor and a single variable capacitor. However, the output signals generated by such transducers have the undesirable characteristic of varying in response to “planar shifts” of the diaphragm. Such planar shifts can be caused by factors independent of the pressure of gas 111, such as temperature variations in the ambient environment of the transducer. Different rates of thermal expansion in different parts of the transducer can cause changes in the distance between the diaphragm and the electrode. As is well known, the accuracy and stability of a transducer may be improved by including two variable capacitors in the transducer and by generating the transducer's output signal according to a function of the difference of the two capacitors (e.g., a function of Cinner minus Couter). When the pressure of gas 111 increases, diaphragm 160 flexes, or bows, so that the central portion of diaphragm 160 moves closer to electrode 122 than do the outer portions of the diaphragm. This causes both the inner and outer capacitances to change, but the inner capacitance changes by a greater amount. The delta between the inner and outer capacitances gives a good indication of the pressure of gas 111. However, if the entire diaphragm 160 moves in a direction normal to the diaphragm, either closer to, or further away from, electrode 122 (i.e., if the diaphragm 160 experiences a “planar shift”), the inner and outer capacitance will change by the same amount (as long as the areas of the inner and outer conductors are equal), and the output signal (which is based in the difference between the two capacitances) will be unaffected. Thus, including two variable capacitors can advantageously render the transducer insensitive to planar shifts of the diaphragm.
As noted above, contaminants (e.g., produced by etching aluminum) are often contained in the gas 111. When such contaminants become deposited on diaphragm 160, they can adversely affect the accuracy of transducer 100. The most common problem caused by contaminant deposition is generally referred to as a “zero shift”. The output signal generated by transducer 100 generally lies in a range between some minimum and maximum values. For example, one popular choice is for the transducer's output signal to be an analog signal that ranges between zero and ten volts, zero volts representing the minimum limit of pressure detectable by the transducer, ten volts representing the maximum pressure detectable by the transducer, and the signal varying linearly with pressure between zero and ten volts. Electronics (not shown), normally disposed in the transducer outside of chambers 120, 130, normally generate this output signal. When a transducer experiences a zero shift, it will no longer generate an output signal equal to zero volts when the pressure of gas 111 is at the minimum limit of detectable pressure. Rather, when the gas pressure is at this minimum limit, the transducer will generate a non-zero output signal. In an effort to reduce zero shifts and other problems caused by contaminant deposition, prior art transducers have used a variety of filters to prevent contaminants from becoming deposited on diaphragm 160.
In the illustrated transducer 100, the contaminant filtration mechanisms 108 include a particle trap system 140 and a baffle 150. Trap system 140 includes a baffle 141, a top view of which is shown in FIG. 2. Baffle 141 includes a central, circular, closed portion 142 and an annular region, defining a plurality of openings 144, disposed around closed portion 142. Openings 144 are formed as series of sectors evenly spaced about the baffle 141 in a circumferential direction, and are also arranged at different diameters radially. The diameter of central portion 142 is greater than that of inlet tube 104 and thereby blocks any direct paths from inlet tube 104 to the diaphragm 160. So, any contaminant in inlet tube 104 can not follow a straight line path all the way to diaphragm 160 and must instead, after traveling the length of inlet tube 104, then travel in a direction generally perpendicular to the length of inlet tube 104 (the perpendicular direction being generally illustrated in FIG. 1B by the arrow L), enter an annular chamber region 146, and then pass through one of the peripheral openings 144. The peripheral openings 144 are sized to prevent relatively large particles (e.g., 250 microns and larger) from passing through the openings. Trap system 140 also includes the chamber 146, which is defined between baffle 141 and housing member 102a. Particles that can't pass through openings 144 tend to accumulate in, or become trapped in, chamber 146.
As noted above, transducer 100 also includes a baffle 150 to further reduce the number of contaminants that can reach the diaphragm 160. Baffle 150 is described in U.S. Pat. No. 6,443,015. FIG. 3 shows a top view of baffle 150. As shown, baffle 150 is essentially a circular metal plate with a plurality of evenly spaced tabs 152 disposed about the circumference. Housing member 102a has stepped regions that come in contact with tabs 152 so as to support baffle 150 in the position shown in FIG. 1B.
Tabs 152 essentially define a plurality of annular sectors 154 (shown in FIGS. 1B and 3) having a width in the radial direction between the peripheral edge of baffle 150 and housing member 102a that is determined by the length of the tabs. Baffle 150 and housing member 102a define a region 158 through which any contaminant must flow if it is to travel from inlet tube 104 to diaphragm 160. The region 158 is annular and is bounded above by baffle 150 and below by either baffle 141 or lower housing member 102a (where the terms “above” and “below” are with reference to FIG. 1B, but do not imply any absolute orientation of transducer 100). Contaminants may enter region 158 via the peripheral openings 144 and may exit region 158 via the annular sectors 154 (shown in FIGS. 1B and 3) between the peripheral edge of baffle 150 and housing member 102a. 
Region 158 is characterized by a length L and a gap g. The length L of region 158 (shown in FIG. 1B) is the distance between openings 144 to annular sectors 154. The gap g of region 154 is the distance between baffle 150 and housing member 102a. The aspect ratio of region 158 is defined as the ratio of the length L to the gap g. As taught in U.S. Pat. No. 6,443,015, the aspect ratio is preferably greater than 10. The length L is preferably at least 1 cm, and preferably in the range of about 1-4 cm; the gap g is preferably no more than about 0.1 cm, and preferably in a range of about 0.025-0.1 cm.
When the pressure in chamber 130 is relatively low (e.g., less than 0.02 Toff), movement of material in chamber 130 is characterized by “molecular flow”. In molecular flow, molecules in chamber 130 generally travel in straight line paths until colliding with a solid surface of the transducer. This stands in contrast to behavior in denser gasses in which molecules are unlikely to travel in straight line paths from one surface of the transducer to another and are instead far more likely to rebound off of each other. Under molecular flow conditions, any contaminant traveling through region 158 will likely collide with the surfaces of baffle 150 and housing member 102a many times prior to reaching, and passing through, an annular sector 154. The probability that a contaminant particle will become deposited on, or stuck to, a surface of baffle 150 or housing member 102a rather than continuing on through region 158 and passing through an annular sector 154 is an increasing function of the number of collisions the contaminant makes with the surfaces of baffle 150 and housing member 102a. Selecting the aspect ratio of the length L to the gap g to be greater than 10 ensures that any contaminant traveling through region 158 is likely to become deposited on a surface of either baffle 150 or housing member 102a rather than continuing on through region 158, passing through an annular sector 154, and ultimately reaching the diaphragm 160.
The use of trap system 140 and baffle 150 has been effective at greatly reducing the number of contaminants that reach the diaphragm 160 and in reducing corresponding zero shifts. However, it would nonetheless be advantageous to provide improved control over deposition of contaminants on the diaphragm of a capacitive pressure transducer.